1. Technical Field
The invention is directed to removal of dissolved gases from hydraulic fluid in fluid power systems, in particular for hydraulic hybrid vehicle applications.
2. Description of the Related Art
Some fluid power systems use hydraulic accumulators. Hydraulic accumulators store hydraulic fluid that is pressurized against a gas cushion. The gas is physically separated from the fluid by a structure, such as a flexible bladder or a sliding piston. The most commonly used gas is nitrogen, due to its low cost and relatively inert properties. Unfortunately, all known bladder materials are somewhat permeable to nitrogen. This causes the fluid in any bladder accumulator to become contaminated with dissolved nitrogen over time, leading to degraded performance of the hydraulic system. Piston accumulators can also be susceptible to nitrogen contamination due to gas permeation across the seal of the piston. Gas permeation tends to increase with higher operating pressures, and is a significant problem at the high pressures found in hydraulic hybrid vehicle systems (e.g., 5,000-7,000 psi).
It can therefore be important to manage nitrogen contamination in a high pressure hydraulic system that employs a hydraulic accumulator. In order to separate dissolved gas from fluid, previously known degasification devices have generally utilized vacuum chambers, swirling/vortex methods, long-delay settling tanks, or mechanical or other external agitation methods to stimulate degasification. A simple, fast, and inexpensive degasification system tailored for a hydraulic hybrid vehicle is needed in the art. While it is possible to simply replace the fluid at periodic intervals, a more practical and less service-intensive solution is desired. For hydraulic hybrid vehicle applications, it would be particularly preferable to devise a fluid degasification system that could be carried on-board the vehicle by which dissolved nitrogen could be continuously or periodically removed and maintained within acceptable limits. Because hydraulic fluid tends to form foamy bubbles when dissolved gas is rapidly released from it, a fluid degasification device preferably will also provide for efficient separation of the resulting foam into its gas and liquid components.